LED backlighting is widely used for LCD TVs and monitors where multi strings of LEDs are mounted either at edge or directly on the back of screens. Typically, one LED string may contain 100 LEDs coupled in series.
FIG. 1 is a schematic circuit of a prior art LED driving system 10 which uses one LED driver to drive n LED strings, wherein n is a desired string number, e.g. n=6. The LED driver adopts linear current sources to balance the LED current between different LED strings. The LED driver includes a power stage to provide an output voltage to drive the LED strings 15_1, 15_2, . . . , 15—n. Each LED string 15—i is series coupled with a corresponding switch 16—i and a corresponding resistor 17—i between the output port of the power stage and a reference ground, wherein i is an integer from 1 to n. The power stage comprises an inductor 11, a diode 12, a switch 13, and an output capacitor 14 connected as shown. The LED driver further comprises a controller 18 which controls the switch 13 to operate in the switch mode, and controls the switch 16—i operate in the linear mode. Through such control, each LED string is regulated to be in the desired mode of operation.
However, the efficiency of the LED driving system 10 is low due to the linear operation of switches 16_1˜16—n, which also causes thermal and long term reliability issues for large screen applications.
FIG. 2 illustrates a schematic circuit of another prior art LED driving system 20 which uses n LED drivers to drive n LED strings, wherein n is a natural number. Compared to the LED driving system 10, the LED driving system 20 has higher efficiency. However, the LED driving system 20 needs many inductors and controllers, which results in higher cost.